Segmented wafer polishing pad

ABSTRACT

Working plate ( 10 ) is formed of forty-eight sections ( 10 ′), with groove ( 12 ) extending through the majority of the thickness of the plate. In one form, dovetail grooves are machined to the back of the sections of the plates ( 10 ′) attached to the base plate ( 19 ). In an alternative arrangement, fixing rails are attached to the backs of the sections and the dovetail grooves are in the base plate ( 19 ).

[0001] The present invention relates to semiconductor wafer manufacturing.

[0002] The present invention relates especially to flat lapping applications using multi-groove slurry distribution.

[0003] Silicon wafers for the computer chip industry are produced by slicing a pure silicon billet into raw wafers, typically 50 to 200 mm in diameter, and 0.0025 to 0.25 mm in thickness. This operation leave saw marks, other surface damage and minor inaccuracies in the thickness. These marks are removed by a process know as “double sides lapping”.

[0004] The size of the wafers have, with the advancement of chip technology, increased in diameters from 100 mm to 300 mm without a change in thickness. All sizes are in current production within the industry.

[0005] In the conventional manufacturing technique, two opposing rotating plates sandwich the wafer for various cycle times, but typically about 12 minutes. A fluid carrying a cutting medium is fed between the plates so that the action of plate rotation exposes the wafer to the cutting medium. This removes the saw marks and reduces the total thickness variation of the wafer to acceptable limits.

[0006] To maximise the flow of fluid across the plate, a series of grooves in a cross-hatch pattern are produced in the cast iron plates. These grooves typically vary in width from 1.5 to 3 mm and in depth from 13 to 25 mm, the plates themselves varying from 50 to 80 mm in thickness.

[0007] The opposing plates are know as the “upper” and “lower” plates whilst the wafers are held in “carriers”.

[0008] During production of wafers, the cutting medium wears away the plates so that, once the operational minimum groove depth has been reached, full replacement plates are required. This is usually in approximately 18 week cycles but is dependent on wafer production levels.

[0009] Much of the prior art considered in relation to this case relates to polishing operations and therefore is wholly irrelevant to the present invention, for example Japanese patent specification JP 2000246622 and U.S. Pat. Nos. 5,718,620, 5,888,121 and 6,019,672.

[0010] Also, other prior art documents relate to surface grinding operations, for example U.S. Pat. No. 4,918,872 and are likewise irrelevant to the present invention. European Patent specification no. 0478912 A relates to a lapping machine in which elements of the grooves are shown in FIG. 4 to be of generally radial configuration. Thus the lapping and polishing disc is constructed by the attachment of segments to the carrier plate by the use of a T-slot mechanism (see FIG. 2), the segments being arranged in concentric rings around the plate, separated from its adjacent one by a small space.

[0011] The construction of such a plate also results in radial and concentric grooves between the segments, these being circumferentially continuous but generally discontinuous in the radial direction.

[0012] According to the present invention, there is provided lapping apparatus for flat lapping applications using multi-groove slurry distribution, the apparatus comprising means to rotate a lapping plate, the lapping plate comprising a base plate having detachably mounted thereon a working plate for abutment against material during the lapping operation, the working plate being rigid and having a cross-hatch arrangement of grooves on the surface for abutment against a semi-conductor wafer in the lapping operation, the cross-hatch arrangement of grooves comprising two sets of continuous grooves.

[0013] According to the present invention, there is also lapping apparatus for the manufacture of semi-conductor wafers, the lapping apparatus comprising means to rotate a lapping plate, said lapping plate comprising a base plate having detachably mounted thereon a working plate for abutment against a semi-conductor wafer during the lapping operation, the working plate being rigid and having a cross-hatch arrangement of grooves on the surface for abutment against a semi-conductor wafer in the lapping operation, the cross-hatch arrangement of grooves comprising two sets of continuous grooves.

[0014] The method of radial and concentric grooves has been utilised within the existing industry, typically on wafers of less than 100 mm in diameter, usually formed in quartz.

[0015] Utilisation of this method for larger wafers can result in potential damage to wafers, and their subsequent loss. The industry current preference is for the use of cross hatch grooves, not only for the reason stated above, but also due to the difficulty associated with cleaning of them, it not being possible to “run through” the grooves in a continuous manner to remove debris contained within them.

[0016] In order to maintain the structural integrity of the plate, a first method selected for the present invention involved the attaching of dovetails to the surface of the plate, removing the requirement for substantial amounts of machining/material removal, when compared to EP0478912. The T-slot design, as indicated within FIG. 2 of EP0478912, does not appear to provide any adjustable mechanical lateral location, apart from that gained by machining of the components to high tolerances, this being expensive. This also gives rise to minute movements of each segment, this causing identifiable surface defects on the surface of the wafer. The present invention may enable machining at greater tolerances by utilising a series of small clamping screws, which act onto the fixing rail via a pressure distribution strip. This takes up relatively large dimensional variations in manufacture, whilst also eliminating any backlash or movement caused by gaps created by these dimensional variations. The pressure imparted onto the side of the dovetail strip via the screws, also imparts, due to the strips geometry, a downward pressure of the working segment onto the mounting plate. This further enhances the location of the segment whilst in operation by the increase in lateral forces from frictional effects. The fact that the present invention is constructed by each segment section butting up to its adjacent one, improves the overall stability of the segmented surface, the segment being able to be supported either side from its neighbours.

[0017] The present invention has been further developed to enable clamping of the segments in an easier manner, reducing the amount of machining required to each segment, and hence improving the stability and integrity of the overall structure.

[0018] This development potentially further reduces the installation and/or changeover time between the working components at the end of their life, since it is now not necessary to remove the segment over the whole length of the dovetail, this possibly being almost equivalent to the annular dimension of said plate, typically over 400 to 500 mm. The development requires only the movement of the segment over a distance of as little as 20 mm.

[0019] A difference of the present invention over EP0478912 is that the section is radially continous, ie. a single part, whereas EP0478912 is formed of a number of sections. This will substantiially increase machine downtime, and the level of accuracy required for assembly.

[0020] Also for the majority of the mentioned prior patents, for example. EP0478912, JP2000246622 & U.S. Pat. No. 4,918,872, that the grooves are formed by the placement of the blocks on the surface of the baseplate, as distinct from the present invention in which the grooves are formed within the segment itself, and do not progress through in depth to the baseplate.

[0021] In a further embodiment of the present invention, there may be provided a clamping force by means of a different medium, for example pnuematic or hydraulic, actuated by a simple cylinder which is integrated into either of the two mating parts. This potentially results in a mechanism which is more suited to the operating environment, reducing the exposure of small components to slurry deposit and/or other potential problems.

[0022] The working plate may be segmented to form a plurality of equi-planar sections (for example segmented in strips, or in columns and rows). The sets of grooves may be mutually orthogonal.

[0023] Preferably, there are two lapping plates each having a base plate with a working plate detachably mounted thereon, the working plates abutting opposing faces of a semi-conductor wafer during the lapping operation.

[0024] The lapping plate may have at least one fixing rail attached to the base plate to effect clamping of the rail at the edges.

[0025] Preferably, the or each working plate is formed of a plurality of sections, the sections of a plate being joined at lines which are preferably groove lines of the working plate when complete.

[0026] Preferably the base plate is formed of one of the following materials:

[0027] Stainless steel, treated aluminum, treated mild steel, gray cast iron, Kevlar/epoxy composite, carbon/epoxy composite, glass/epoxy composite, glass/polyester composite.

[0028] Preferably, the working plate is formed of one of the following materials:

[0029] Spheroidal cast iron, ceramics.

[0030] The present invention also provides a working plate having means to attach the plate onto a base plate of a lapping apparatus the working plate having a cross-hatch arrangement of grooves on the surface for abutment against a semi-conductor wafer in the lapping operation, the cross-hatch arrangement of grooves comprising two sets of mutually orthogonal grooves. The working plate may include one or more of the features defined in the present invention described herein.

[0031] The present invention also provides lapping apparatus for working on a wafer material, the apparatus comprising a lapping plate having a plurality of recesses in the surface of the lapping plate remote from the wafer material.

[0032] The lapping plate may comprise a working plate and a base plate, or it may comprise a single plate.

[0033] Preferably, the plurality of recesses are distributed uniformly over said remote surface.

[0034] Preferably, the plurality of recesses comprise a substantial portion (e.g. 30 to 40%, preferably 35 to 38%) of the original, non-recessed plate volume, also being 50% to 60% of the final or actual volume.

[0035] Preferably, the recesses are arranged in a radial pattern or an orthogonal pattern. Preferably, the grooves are likewise so arranged.

[0036] The present invention relates especially but not solely to apparatus for the manufacture of semi-conductor silicon wafers, but is also applicable to manufacture of other types of materials, e.g. sintered metal and quartz.

[0037] In order that the invention may more be readily understood, a description is now given, by way of example only, reference being made to the accompanying drawings, in which:

[0038]FIG. 1 shows a typical general arrangement of working plate embodying the present invention;

[0039]FIG. 2 shows a section of a working plate;

[0040]FIG. 3 shows a view along arrows 3-3 of FIG. 2;

[0041]FIG. 4 shows a view along arrows 4-4 of FIG. 2;

[0042] FIGS. 5 to 7 show corresponding views of another section of working plate;

[0043] FIGS. 8 to 10, 11 to 13, and 14 to 16 show corresponding views of the three further sections;

[0044]FIG. 17 shows assembling of sections of the working plate;

[0045]FIG. 18 shows clamping details of a working plate;

[0046]FIG. 19 shows an alternative working plate;

[0047]FIG. 20 shows part of a working plate and base plate;

[0048]FIG. 21A and B show variants of the arrangement in FIG. 20;

[0049]FIGS. 22A and B shows a conventional plate before and after use;

[0050]FIGS. 23A, B and C shows a plate of the present invention before and after use;

[0051]FIG. 24 shows the underside of a base plate;

[0052]FIGS. 25 and 26 show an alternative form of clamping;

[0053] FIGS. 26 to 28 show parts of a radial segmented form of a plate.

[0054] There is shown in FIG. 1 one quadrant of a working plate 10 which has a plurality of grooves 12 in a cross-hatch arrangement on the surface which contacts the semi-conductor silicon wafer in the lapping operation. Working plate 10 is formed of forty-eight sections 10′, there being five different types of section, plan views of which are shown in FIGS. 2, 5, 8, 11, and 14. As can be seen in, for example FIG. 3, groove 12 (typically 20 mm deep) extends through the majority of the thickness of plate 10 (typically 30 to 35 mm). The thickness of plate 10 represents solely the working or grooved part of a conventional lapping plate plus the plate thickness used to mount the working plate (see FIG. 22A to C).

[0055]FIG. 1 indicates, by the use of numerals in circles, the sequence of assembling together the five types of section 1, 2, 3, 4, 5 on a base plate (not shown) permanently fixed on a lapping machine to form the illustrated quadrant of plate 10, the process being repeated until the entire plate 1 is produced.

[0056] Dovetail grooves are machined to the back (i.e. the side adjacent to the base plate) of the sections of the plates 10′ (see FIG. 20) with dovetail 95 attached to base plate 19. In an alternative arrangement, fixing rails are attached to the backs of the sections and the dovetail grooves are in the base plate 19 (see FIG. 21A). By means of a load distribution strip (not shown), the rail is clamped from the edges, having pressure applied by a series of screws, these being located in the base of the second layer component. The plate 10, having clearance between the top of the dovetail and its underside, is hence located horizontally and vertically by means of the dovetail angle, and longitudinally by the friction generated at the clamp.

[0057] In another embodiment (see FIG. 21B), additional rails 212 are provided in place of machined grooves. Further embodiments comprise combinations or reversals of the described embodiments.

[0058]FIG. 17 shows the situation in which completion of assembly of three quadrants has been achieved, except for section 20 which is shown being slid along fixing rail 21. In FIG. 17, the sections of the last quadrant 30 of working plate 10 are shown above their final position.

[0059] Each plate 10 is made up of sections 10 ¹ of a size enabling ready handling, the sections preferably joining at the existing plate groove lines. Once the section has worn down, it is unscrewed and replaced. The positioning of these screws is such as to not interfere with the through working depth.

[0060]FIG. 18 shows a fixing rail 21 secured on base plate 19 by screw 22 with working plate section 10′ held on fixing rail 21 by its machined dovetail profile and secured in position by grub screw 23 and clamp strip 24.

[0061] In variants, the fixing rail profile is not dovetail but is ovoid or elliptical in cross-section, or any other appropriate shape.

[0062] The fastening methods utilised are intended for attachment with the base plate in situ.

[0063] The method of construction using a removable working surface is also applicable with attachment from the rear of the base plate, for example fixing screws, T-bolts, etc.

[0064] The current method outlined is proposed as attachment of the working surface is made from the working side of the plate, eliminating the need for removal of the base plate from the machine. It is also a “low profile” arrangement thus reducing the amount of intrusion of the fixing method into the overall thickness of the plate.

[0065] While attachment from the rear of the plate may not necessarily reduce downtime through non-removal of the backplate, the benefits of using dissimilar materials, i.e. performance, weight reduction, are still achievable.

[0066] Alternative fastening methods for retention of the working surface could also include, but are not restricted to:

[0067] Magnetic clamping, though not necessarily applicable to the production of silicon wafers;

[0068] Male/female arrangements incorporating the plate as one half of the attachment. Dovetail or grooves machined into the surface of the plate;

[0069] Male/female arrangements whereby other mechanical fixings are utilised;

[0070] Other mechanical means of attachment, cotter or dowel pins, taper pins, circlips;

[0071] Adhesively bonding the working surface to the base plate, either permanently or temporarily;

[0072] Use of other fastening media, e.g. Velcro (RTM) or adhesive tapes.

[0073] The clamp section may be a standard proprietary dovetail section produced in hardened rust protected stainless steel. The dovetail and side surface are precision ground for high location accuracy. The standard product enables quick and easy replacement of damaged sections should this occur. Slotted holes enable each rail to be set up accurately independent of the accuracy of the mounting holes.

[0074] Plate Dimension examples are: Thickness Groove Working Plate (Overall) Depth Thickness 62.5 20 33-40 50 20 33-35 60 20-25 30-35 50.8 16.5 28-32 50.0 20.0 30-35 60.0 18.0 30-35 52 14 26-31

[0075] Advantages of the illustrated embodiment include:

[0076] simplifying the handling as compared to conventional lapping plates which are difficult to handle in the working environment of wafer production due to their size and weight.

[0077] Substantially reducing the time involved in plate changes which can take up to 2 days for conventional lapping plates and machines, i.e. representing 2 days lost production.

[0078] Avoiding the need for large labour teams required for plate changes in conventional machines, as the present invention requires fewer people to effect plate changes.

[0079] Reduced environmental problems from disposal of the worn out plates by the smaller sections.

[0080] On site storage space is reduced for future replacements.

[0081] Plate cleaning is easier by stripping down the sections and replacing with new which allows a thorough cleaning. Cleaning off the cutting medium between shifts is time consuming and hostile to the working environment.

[0082] The present invention provides a modular constructed working plate with individual sections for the working surface that are substantially smaller and lighter than the lapping plates with existing systems.

[0083] Thus, working plates are more easily handled and can be removed without the requirement for specialist equipment, e.g. cranes or fork-lift trucks.

[0084] This working plate with small section also enables smaller machines to be employed at the manufacturing stage for grooving. Overall flatness of the working plate (e.g. less warping) is also improved because of the smaller size, this reducing the incidence of dimensional variations or warping in thickness of the plate. As a consequence, the initial conditioning of the lapping machine prior to production will be reduced.

[0085] In an embodiment, the base plate is integrated into the lapping machine, replacing the current lifting ring/backing plate. This can either be employed to reduce machine loading or could result in a single stiffer overall component of the same original two component thickness. A further advantage of this system is that slurry pipes may be permanently attached to the rear to the base plate.

[0086] The separate plate and base plate enables the two to be made of dissimilar materials, such that the material of each plate can be chosen to optmise its own distinct and particular function.

[0087] The base plate may be constructed from a variety of materials, such as composites, stainless steel, Aluminium or Mild Steel, or Grey Cast Iron, though the material used is not limited to these.

[0088] This has certain advantages, in that the material can be selected for its reduced cost, or properties independent of those required for the working plate. The use of stainless steel and composites provide improved anti-corrosion properties, rusting of slurry holes and its subsequent carriage to the wafers via the slurry being a problem prevalent in current plate construction. To a lesser extent Aluminium and Mild Steel, employing anodising, painting or other surface treatment procedures can also be employed for the base plate. This property is desirable, especially since the base plate is mounted for longer periods of time when compared to existing systems. A further advantage of being able to utilise different materials is the ability to select those having higher thermal conductivity, rear base plate, if becoming integrated into the machine could contain cooling ducts, this now being in closer proximity to the working surface, providing a more rapid response to demand.

[0089] Composites and Aluminum can be selected for machine isolation capabilities through improved damping characteristics.

[0090] Aluminum and Composites can also achieve a lower base plate weight, reducing machine loading and possibly eliminating the requirement for specialist equipment at initial mounting.

[0091] Details of the material combinations and advantages are as follows for the base plate: Material Benefits Keviar/Epoxy composite improved damping characteristics, reduced weight; Carbon/Epoxy composite increased stiffness to weight ratio. Potential for tailoring thermal expansion properties reduced weight; Glass/Epoxy composite improved corrosion resistance reduced weight; Glass/Polyester composite improved corrosion resistance reduced weight. Aluminium Reduced weight/improved damping Stainless Steel Corrosion resistance Grey Cast Iron Improved heat absorption over existing materials, improved rigidity

[0092] Details of the materials and advantages for the working plate are as follows: Material Benefits Spheroidal Cast Iron Existing material. Used for low cost and ability to retain abrasive medium within nodule sites, improving cutting. Ceramics Heard wearing, often castable.

[0093] The lower overall masses will also provide power saving benefits.

[0094] While the initial cost of the system may be higher than conventional system, subsequent costs for replacement working surface components are lower, and hence overall costs are reduced in the long term, this in turn reducing overall inventory costs.

[0095] Since the base plate is permanently attached to the machine, it is not necessary to re-align after each plate change. This leads to a reduction in machine downtime, and associated set-up costs.

[0096] The working plates are more easily handled and can be removed without the requirement for specialist equipment, for example cranes or fork-lift trucks, or larger numbers of people, again further reducing the swap out cost.

[0097] The employment of two sets of surface components enable the downtime of the machine to be reduced significantly by requiring less time for the current working set being removed, the mounting plate cleaned to enable attachment of the second set, and then re-conditioning and returning to production. This first set now can be cleaned remote to the production environment, with cleaning now becoming a simultaneous operation with production, rather than sequential as previously for this operation. Since the parts are substantially smaller, cleaning can be undertaken by immersion in a ultrasonic bath.

[0098] In conventional methods of whole plate replacement currently used, i.e. mounting and working surface are one and the same component, when the plates come to the end of their working life, the whole of the plate remaining is discarded. This, in certain designs, results in up to 76% of the plate's thickness being thrown away.

[0099] In the method proposed, the mounting plate is retained and only the remaining portion of the working surface is discarded. Hence as little as 20-30% approximately is now discarded dependent on the plate design utilised.

[0100]FIG. 19 is a plan view of part of another working plate 25 in which the grooves 26 are arranged in another cross-hatch pattern.

[0101] In a further variant, sections of the base plate on the side adjacent to the lapping machine are removed. Thus pockets on the rear face (i.e. remote from the working plate) are removed on components over a greater extent, i.e. the majority of the face.

[0102] Plates currently in existence have only small pockets removed around the periphery, in order to eliminate the mass of the unsupported area of the plate.

[0103] In the new construction, the pockets extend over the whole of the rear face, in order to:

[0104] Reduce material wastage at plate life end;

[0105] Lower shipping costs;

[0106] Improve the overall stiffness to weight, reducing plate droop;

[0107] To provide a more even pressure distribution across the lapping surface.

[0108] Such modification is applicable to:

[0109] Existing plates produced from monolithic castings;

[0110] The rear structure of the mounting plate of this illustrated embodiment;

[0111] The rear structure of the working surface component(s).

[0112]FIG. 22A shows a conventional plate 100, before use, of thickness 50 mm with a groove portion 101 of groove depth 17 mm. FIG. 22B shows the plate 100 at the end of its usable life with the groove depth 101′ reduced to about 2 or 3 mm.

[0113]FIG. 23A is a side elevation of a plate embodying the present invention with working plate 110 incorporating grooves 111 and dovetail 112, also base plate 113; FIG. 23B shows the end elevation. FIG. 23C shows, in side elevation, the base plate 113 and the remainder of the working plate 101′ at the end of its usable life, the wasted portion of the plate being about 14 mm, with the minimum groove depth being 2 or 3 mm.

[0114]FIG. 24 shows the underside of part of base plate 113 with the radially aligned recesses 114 in view, these recesses constituting 55% of the total original volume of the base plate. In this way, substantial reduction of weight of the base plate is achieved, likewise significant materials savings are achieved with appropriate cost benefits.

[0115]FIGS. 25 and 26 show details of an alternative clamping arrangement, utilising a “keyhole” profile to develop the design, still further, with now only minor machining being required to either the base plate or the working plate components.

[0116] The method of clamping is produced by the use of a high rate disc spring 201, which is located around a central bobbin arrangement attached to the underside of the working plate 203. Also located around the central pin is a secondary bobbin 205, concentric to the first, which is actuated by the spring pressure, towards the working plate itself. This produces a clamping action, by which once inserted and located on the mounting plate 203, will retain the working plate in position. The mounting plate 203 has machined within it the keyhole profile 206, which due to its geometry, lifts the outer bobbin 205, and locates it centrally within the keyhole profile 206. Due to the high loads involved by the use of a number of these per segment, it is necessary for some mechanical method to be employed for the attachment and removal of the working plate 203.

[0117] The method employed utilises a simple screw arrangement, which enables the plates to be wound on and off the mounting plate 203.

[0118] Detailed within EP0478912, (see FIG. 1) the blocks are shown as attached to the carrier plate by means of a fixing screw, 54, this being fixed from the rear face. This has the disadvantage of it being necessary to remove the carrier plate from the machine in order to remove the blocks. The removal of the carrier plate, and then the subsequent removal and re-assembly of the blocks, would not purvey any benefit in terms of potential machine downtime, and is likely to be of longer duration when compared to the current single plate construction.

[0119] The design philosophy for the present invention has always been that the mounting plate remain fixed to the machine at all times, and only the individual blocks of the working surface be removed, these being substantially smaller and lighter in weight, and hence significantly more handleable in the environment intended.

[0120] Plate Profile Stability

[0121] In the prior art, it is known that the plates themselves wear differentially across their surface from the inside diameter to the outside diameter, resulting in a change in profile of the plate.

[0122] This change in profile is detrimental to the finished requirements of the wafers produced, both as regards their overall shape, and in relation to their total thickness variation, TTV.

[0123] The construction of the present invention provides the manufacturer/operator with the opportunity of substituting areas of the plate with either the same material, or a different material, both of whose properties have been selected to provide greater plate profile stability.

[0124] In FIGS. 26 to 28, there is shown a single radial sector 230, a base plate 231 with an inner face 232 comprising the inside diameter and an outer face 233 comprising the outside diameter and grooves 234.

[0125]FIG. 27 shows two adjacent sectors 230 and 240, while FIG. 28 shows groove width gw, groove depth gd, segment depth sd and baseplate depth bd.

[0126] The present invention also provides methods of manufacturing such plates, methods of assembling such plates and methods of using such plates and lapping apparatus. 

1. Lapping apparatus for flat lapping application using multi-groove slurry distribution, the apparatus comprising means to rotate a lapping plate, the lapping plate comprising a base plate having detachably mounted thereon a working plate for abutment against material during the lapping operation, the working plate being rigid and having a cross-hatch arrangement of grooves on the surface for abutment against a semi-conductor wafer in the lapping operation, the cross-hatch arrangement of grooves comprise two sets of continuous grooves.
 2. Lapping apparatus for the manufacture of semi-conductor wafers, the lapping apparatus comprising means to rotate a lapping plate, said lapping plate comprising a base plate having detachably mounted thereon a working plate for abutment against a semi-conductor wafer during the lapping operation, the working plate being rigid and having a cross-hatch arrangement of grooves on the surface for abutment against a semi-conductor wafer in the lapping operation, the cross-hatch arrangement of grooves comprising two sets of continuous grooves.
 3. Lapping apparatus according to claim 1 or 2 wherein the working plate is segmented to form a plurality of equi-planar sections.
 4. Lapping apparatus according to any preceding claim wherein the working plate is segmented in strips.
 5. Lapping apparatus according to any preceding claim wherein the working plate is segmented in columns and rows.
 6. Lapping apparatus according to any preceding claim wherein the sets of grooves are mutually orthogonal.
 7. Lapping apparatus according to any preceding claim, comprising two lapping plates each having a base plate with a working plate detachably mounted thereon, the working plates abutting opposing faces of a semi-conductor wafer during the lapping operation.
 8. Lapping apparatus according to any preceding claim comprising at least one fixing rail attached to the base plate.
 9. Lapping apparatus according to claim 8 wherein the fixing rail has a dovetail profile.
 10. Lapping apparatus according to claim 8 wherein the fixing rail has an ovoid profile.
 11. Lapping apparatus according to claim 9 or 10 wherein the at least one rail comprises a load distribution strip to effect clamping of the rail at the edges.
 12. Lapping apparatus according to any preceding claim wherein the or each working plate is formed of a plurality of sections.
 13. Lapping apparatus according to claim 12 wherein the sections of a plate are joined at lines which are groove lines of the working plate when complete.
 14. Lapping apparatus according to claim 1 wherein the working plate includes one or more further grooves extending generally diagonally relative to the arrangement of grooves.
 15. Lapping apparatus according to any preceding claim, wherein the base plate is formed of one of the following materials: Stainless steel, treated aluminum, treated mild steel, gray cast iron, Kevlar/epoxy composite, carbon/epoxy composite, glass/epoxy composite, glass/polyester composite.
 16. Lapping apparatus according to any preceding claim wherein the working plate is formed of one of the following materials: spheroidal cast irons, ceramics.
 17. Lapping apparatus according to any preceding claim wherein the working plate is detachably mounted on a base plate by means of one or more of the following: Magnetic clamping; Male/female arrangements incorporating the plate as one of the attachment; Male/female arrangements with other mechanical fixings; Other mechanical means of attachment e.g. cotter of dowel pins, taper pins, circlips; Advesively bonding the working surface to the base plate other fastening media, e.g. Velcro (RTM) or adhesive tapes. Hydraulic or Pneumatically operated retention methods.
 18. A working plate for lapping apparatus according to any preceding claim.
 19. A working plate according to claim 14 being rigid and having means to attach the plate onto a base plate of a lapping apparatus and working plate has a cross-hatch arrangement of grooves on the surface for abutment against a semi-conductor wafer in the lapping operation the cross-hatch arrangement of grooves comprise two sets of continuous grooves.
 20. A working plate according to claim 19 wherein the attachment means comprises one or more of the following: Magnetic clamping; Male/female arrangements incorporating the plate as one of the attachment; Male/female arrangements with other mechanical fixings; Other mechanical means of attachment e.g. cotter of dowel pins, taper pins, circlips; Advesively bonding the working surface to the base plate other fastening media, e.g. Velcro (RTM) or adhesive tapes. Hydraulic or Pneumatically operated retention methods.
 21. A working plate according to claim 18 or 19 wherein the working plate is formed of a plurality of sections.
 22. A working plate according to claim 20 wherein the working plate is formed of sections for joining at lines which are groove lines of the working plate when complete.
 23. A working plate according to any of claims 18 to 22 wherein the working plate has a cross-hatch arrangement of grooves on the surface for abutment against a semi-conductor wafer in operation.
 24. A working plate according to any of claims 20 or 21 wherein the working plate is formed of one of the following materials: spheroidal cast iron, ceramics. 